There exists a prior art installation for manufacturing reinforced duroplastic pipes, that is, pipes made of a resin, to which may be added a filler, the appropriate catalyst, as well as glass fibre inserts and a sand in a centrifugal process. This prior art installation has an injection carriage provided with a feed arm and is suspended, in the manner of a stock crane, on a vertically displaceable frame, i.e. on two tracks disposed one on each column, and contains all the starting materials, except the liquid resin, for feeding the rotating, cylindrical mould, namely 11/2 to 3 m.sup.3 sand, the glass fibres and the liquid catalyst. To refill the sand, the carriage must be brought to a special loading point and filled there, which always causes a considerable interruption in the operation. To supply the electrical energy required to drive the travel mechanism, the liquid pumps, the feed device for the sand, the cutting device for the glass fibres, and to feed the latter into the interior of the mould, towed cables are used for each of the two displacement devices which are expensive to install and maintain, but which cannot be replaced by anything less expensive, are used for each of the two displacement devices.
When a pipe is manufactured, the individual components which go to make up the pipe, that is, the liquid resin, which may contain a fine filler, the corresponding catalyst, the glass fibers and the sand, are introduced from an outlet at the free end of the feed arm into a rotating, drum-shaped mould at an appropriate mould speed, injection carriage advancement rate and quantity of individual components conveyed are selected according to the required composition. During careful quality checking of the pipes manufactured with these prior art installations, small fluctuations in the uniformity of the composition of the wall were noted, consisting essentially of an irregularity in the distribution of the sand, both as regards the thickness and the graining. Although these fluctuations lie below a tolerance level and are therefore harmless, they ought to be avoided to improve quality. However, since they could not be systematically reproduced, it was not possible to establish the cause and to eliminate them. Detailed studies and intermediate measurements revealed that the fluctuations in the regularity of the sand granulation, that is the accumulation of course grained sand at certain points and of fine grained sand at other points are probably caused by the sand separating out in the sand container due to the vibration of the container, and that the cause of the irregularities in the distribution of the sand might be due to the fact that the height of the feed arm outlet of the injection carriage is not constant.